1/2" T45 tubing from ProFormance. That's what I use - see wishbone in photo above ^^^^. Not expensive, easy(ish) to bend, and generally easy to work with.
P.S. I'm about to order a new stash so if you want some, let me know. I usually order 1m lengths to have some wiggle room.
Woooooooaaaah there, overkill. Get with the slender seatstays vibe, dude. Vertical compliance, dontchaknow.